ci: cached bgfx example build job (#286)

[apotema]

Closes #286

What

Adds a dedicated, cached CI job bgfx-example to .github/workflows/ci.yml that generates and builds examples/bgfx, so bgfx template/codegen regressions are caught by a real build. PR #284 fixed bgfx's lifecycle template and example but had to leave the backend un-gated because its C++ compile runs 50+ minutes cold — far too slow for the shared examples-integration job.

Job design

• pull_request-only (if: github.event_name == 'pull_request'), ubuntu-latest, timeout-minutes: 90 to cover the cold case.
• Same sibling-checkout recipe as the examples-integration job (labelle-gui, labelle-cli + its zig-pkg/labelle-gui unpack path, labelle-core/engine/gfx at main), same apt deps, same mlugg/setup-zig@v2 / Zig 0.16.0.
• actions/cache@v4 persists the generated project's LOCAL zig cache, labelle-assembler/examples/bgfx/.labelle/bgfx_desktop/.zig-cache — mlugg/setup-zig already caches the global zig cache, but the expensive bgfx/bimg/bx C++ objects land in the example's local cache.
• Key: bgfx-example-${{ hashFiles('labelle-assembler/backends/bgfx/**') }} so backend changes roll the key, with a bgfx-example- restore-keys prefix fallback so warm runs still restore the bulk of the compiled C++ objects even when the key changes and only the delta rebuilds.
• The cache is restored before labelle generate, which rewrites generated sources but leaves .zig-cache intact.
• Also updates the stale comment on the wgpu example step that said bgfx was un-gated.

Verification

This PR's own CI run is the proof: the new bgfx-example job runs on this PR. Note the first run is a cold ~50 min build (no cache exists yet); subsequent PR runs restore the cache via the prefix fallback and should complete in minutes.

🤖 Generated with Claude Code

[gemini-code-assist]

Note

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[cursor]

PR Summary

Low Risk
CI workflow-only change; no runtime or product code paths affected.

Overview
Adds a pull_request-only bgfx-example job that runs labelle generate and zig build on examples/bgfx, so bgfx codegen/template regressions are caught in CI without bloating the shared examples-integration job.

The job mirrors the sibling-repo checkout and build setup used elsewhere, then uses actions/cache@v4 on the generated project’s local .labelle/bgfx_desktop/.zig-cache (keyed on backends/bgfx/** with a prefix fallback) so cold ~50 minute C++ compiles amortize to warm runs in minutes. Cache restore happens before generate so regenerated Zig sources still reuse cached C++ objects. 90-minute timeout covers the first cold run.

The wgpu example step comment is updated to say bgfx is gated by this job instead of still being un-gated.

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[apotema]

Superseded by #295 (merged), which added the dedicated cached bgfx-build job. Closing as redundant — the remaining gap is Android build coverage (the .so/APK are still manual-only), which I'm addressing in a separate Android-NDK CI job.